Place roast in an earthenware bowl with onions and seasonings.
Pour over it a mixture of red wine vinegar and boiling water.
Marinate 3 days or more in refrigerator.
Turn meat twice a day with 2 wooden spoons; never pierce with a fork.
To cook: drain meat and brown thoroughly on all sides in hot fat in heavy skillet.
Add marinade; cover pan and simmer slowly 3 to 4 hr., or until tender.